1. Field of the Invention
The present invention relates generally to implantable temporary medical prostheses that do not require interventional procedures for removal. In particular, the present invention is a disintegrating implantable medical prosthesis that disintegrates into small pieces that are harmlessly transported out of the body by normal body function.
2. Related Technology
Medical prostheses frequently referred to as stents are well known and commercially available. They are, for example, disclosed generally in the Wallsten U.S. Pat. No. 4,655,771, the Wallsten et al. U.S. Pat. No. 5,061,275 and in Hachtmann et al., U.S. Pat. No. 5,645,559. Devices are used within body vessels of humans for a variety of medical applications. Examples include intravascular stents for treating stenoses, stents for maintaining openings in the urinary, biliary, tracheobronchial, esophageal, and renal tracts, and vena cava filters.
A delivery device which retains the stent in its compressed state is used to deliver the stent to a treatment site through vessels in the body. The flexible nature and reduced radius of the compressed stent enables it to be delivered through relatively small and curved vessels. In percutaneous transluminal angioplasty, an implantable endoprosthesis is introduced through a small percutaneous puncture site, airway, or port and is passed through various body vessels to the treatment site. In endoscopy, the delivery device is passed through the instrument channel of the scope. After the stent is positioned at the treatment site, the delivery device is actuated to release the stent. Following release of the stent, the stent is allowed to self-expand within the body vessel, in the case of self-expanding stents, or alternatively, a balloon is used to expand the stent. The delivery device is then detached from the stent and removed from the patient. The stent remains in the vessel at the treatment site as an implant.
Stents must exhibit a relatively high degree of biocompatibility since they are implanted in the body. An endoprosthesis may be delivered into a body lumen on or within a surgical delivery system such as delivery devices shown in U.S. Pat. Nos. 4,954,126 and 5,026,377. Suitable materials for use with such delivery devices are described in U.S. patent application Ser. No. 08/833,639, filed Apr. 18, 1997.
Commonly used materials for known stent filaments include Elgiloy® and Phynox® metal spring alloys. Other metallic materials that can be used for stent filaments are 316 stainless steel, MP35N alloy, and superelastic Nitinol nickel-titanium. Another stent, available from Schneider (USA) Inc. of Minneapolis, Minn., has a radiopaque clad composite structure such as shown in U.S. Pat. No. 5,630,840 to Mayer. Stents can also be made of a Titanium alloy as described in U.S. Pat. No. 5,888,201.
Bioabsorbable stents have also been proposed, for example in U.S. patent application Ser. No. 08/904,467 entitled Bioabsorbable Self-Expanding Stent, filed Aug. 1, 1997, and commonly assigned to the assignee of this application. Such bioabsorbable stents may be formed, for example, from a number of resilient filaments which are helically wound and interwoven in a braided configuration. Such stents assume a substantially tubular form in their unloaded or expanded state when they are not subjected to external forces. When subjected to inwardly directed radial forces, the bioabsorbable stents are forced into a reduced-radius and extended-length loaded or compressed state. Bioabsorbable stents are generally characterized by a longitudinal shortening upon radial expansion.
Bioabsorbable implantable endoprostheses such as stents, stent-grafts, grafts, filters, occlusive devices, and valves may be made of poly(alpha-hydroxy acid) such as poly-L-lactide (PLLA), poly-D-lactide (PDLA), polyglycolide (PGA), polydioxanone, polycaprolactone, polygluconate, polylactic acid-polyethylene oxide copolymers, modified cellulose, collagen, poly(hydroxybutyrate), polyanhydride, polyphosphoester, poly(amino acids), or related copolymers materials, each of which have a characteristic degradation rate in the body. For example, PGA and polydioxanone are relatively fast-bioabsorbing materials (weeks to months) and PLA and polycaprolactone are a relatively slow-bioabsorbing material (months to years).
The implantation of an intraluminal stent generally causes a generally reduced amount of acute and chronic trauma to the luminal wall while performing its function. Stents that apply a gentle radial force against the wall and that are compliant and flexible with lumen movements are generally used in diseased, weakened, or brittle lumens. Such stents are generally capable of withstanding radially occlusive pressure from tumors, plaque, and luminal recoil and remodeling.
There remains a continuing need for stents with particular characteristics for use in various medical indications. Stents are needed for implantation in an ever growing list of vessels in the body. Different physiological environments are encountered and it is recognized that there is no universally acceptable set of stent characteristics. A surgical implant such as a stent endoprosthesis must be made of a non-toxic, biocompatible material in order to minimize the foreign-body response of the host tissue. The implant must also have sufficient structural strength, biostability, size, and durability to withstand the conditions and confinement in a body lumen.
All documents cited herein are incorporated herein by reference in their entireties for all purposes.